Internal antennas are preferred to external antennas for mobile communication terminals because of the preference for the aesthetic aspect of terminals. In order to provide various mobile communication services, such as Bluetooth, WiBro, digital broadcasting, and global roaming services, the demand for internal antennas that operate in multiple bands is increasing.
Representative external antennas are implemented using helical antenna technology, whereas representative internal antennas that operate in multiple bands are implemented using planar inverted-F antenna (PIFA) technology including antenna patterns each including a radiation portion, a ground portion, and a feeding portion formed at a location adjacent to the ground portion and configured such that feeding signals are input thereto.
Meanwhile, because of the demand for high-speed multimedia services using wireless mobile communication technology, interest in antenna technology that is capable of increasing communication capacity in mobile communication systems is increasing.
Technologies that have been proposed to meet the demand include a Multi-Input Multi-Output (MIMO) antenna technology that employs two or more antennas for each of a base station and a mobile communication terminal, transmits data via various paths, and detects signals received via the paths at a receiving end, and a diversity antenna technology that receives signals using two or more antennas and detects received signals by combining output. MIMO/diversity antennas have the advantages of increasing the reliability of transmitted data and overcoming limitations regarding the amount of transmission of mobile communication.
However, such MIMO/diversity antennas are problematic in that an interference phenomenon attributable to electromagnetic wave signals occurs between a plurality of antennas because the plurality of antennas operate in the same frequency band. In particular, when MIMO/diversity antennas operate in a frequency band lower than 1 GHz, the guarantee of isolation between a plurality of antennas becomes the most important issue because of the limited length of a ground surface.
FIG. 1 is a diagram schematically illustrating current components shared on a ground surface by a plurality of planar inverted-F antennas that constitute a conventional MIMO/diversity antenna, and FIG. 2 is a diagram illustrating the isolation of the conventional MIMO/diversity antenna.
As illustrated in FIG. 1, in conventional MIMO/diversity antennas, ground pads 30 corresponding to respective ground portions that constitute a plurality of planar inverted-F antennas 10 and 20 are located in a diagonal direction in order to maximize the physical length of a ground, surface 1 in a limited ground surface structure.
In this case, when the operating frequency of a MIMO/diversity antenna is, for example, about 900 MHz and the MIMO/diversity antenna operates in its adjacent frequency band, the length of a common ground surface 1 is 80 mm, which is λ/4 of the operating frequency.
However, the conventional MIMO/diversity antenna is problematic in that sufficient isolation cannot be ensured in a frequency band lower than 1 GHz, as illustrated in FIG. 2, because an interference phenomenon attributable to electromagnetic waves occurs between the plurality of planar inverted-F antennas because of current components shared on the ground surface between the plurality of planar inverted-F antennas 10 and 20.
Accordingly, there is an urgent need for a practical and useful technology that is capable of providing a MIMO/diversity antenna for minimizing current components shared on a ground surface on a printed circuit board and also preventing electromagnetic interference attributable to electromagnetic waves radiated from a plurality of planar inverted-F antennas, thereby improving isolation.